Conventional land mobile satellite systems (“LMSSs”), such as the SIRIUS SATELLITE RADIO® mobile satellite system, use a constellation of three satellites in 24-hour repeating ground track, highly elliptical orbits (“HEO”) to provide high elevation angle coverage of the continental United States and Canada. The ground track of the orbit disclosed in U.S. Pat. No. 6,564,053 (“the Briskman patent”) is depicted as orbit 101 in FIG. 1. Orbit 101 has an orbit inclination of 63.4°, and a coverage of ±8 hours about the orbit apogee.
The conventional orbit described in the Briskman patent includes a pronounced figure-eight loop over northern Canada, which reduces the elevation angles to the coverage region (depicted as a cluster of asterisks). While the system and orbit described in the Briskman patent provides user-to-spacecraft elevation angles which are significantly higher than typical geostationary earth orbit (“GEO”) systems, the orbit described therein is suboptimal, based on a range of criteria related to system performance. Specifically, and as illustrated in FIG. 2, when the corresponding elevation angles for 125 North American cities are plotted, an elevation droop (denoted by reference 201) is evident when a spacecraft is near apogee, since high latitudes reduce elevation angles to the coverage region, resulting in a lower probability of link closures. The probability of link closure in an urban environment with at least one spacecraft and at least two spacecraft are depicted in the chart at FIGS. 3 and 4, respectively.
Due to their substantial build, launch and operational costs, it is considered highly desirable to overcome the deficiencies of conventional land mobile satellite systems. Specifically, it is desirable to provide an enhanced land mobile satellite system using an optimized orbit which reduces the system cost to achieve a given quality of service, by allowing the use of lower power and lighter satellites, and using fewer terrestrial repeaters or, conversely, allowing improved performance using the highest spacecraft power capability available.